The invention relates to production of passive devices such as electro-cerarmic varistor and capacitor devices.
Varistors are produced by pressing metal oxide ceramic powder such as Zinc Oxide powder. The pressed powder is sintered to provide the body (often referred to as a xe2x80x9cdiscxe2x80x9d) of the varistor, which exhibits the non-linear current-voltage characteristics. Conductive electrodes are applied to the disc and they are cured (or xe2x80x9cfiredxe2x80x9d). Also, a method of producing capacitors involves pressing ceramic powder and sintering the disc, followed by firing electrodes.
For passive devices such as these, it is well recognised that the sintering part of the process is very important for setting the microstructure (grain size and grain boundary chemistry) and the composition. These in turn determine electrical properties such a nominal voltage (Vnom), energy handling capability, clamping performance (varistor), and capacitance (capacitors).
Sintering has typically been performed by conveying the discs through a continuous oven at a temperature typically in the range of 1200xc2x0 C. to 1300xc2x0 C. For varistors, the sintering time is of the order of 18 to 30 hours and this process has effectively imposed limits on Vnom and the clamp ratio. There is also, of course, a high energy consumption.
Use of microwave radiation for sintering has been described in Japanese Patent Specification No. JP 05006808. This document describes a process of sintering compounded and moulded zinc oxide powder by irradiation with microwave radiation for a very short duration.
Regarding outer electrodes, these are formed on the discs by thick film deposition of the electrode material, typically thick film silver or a silver-palladium alloy mixed with 4-6% glass frit and a 25-30% organic binder-solvent mix. The film is then dried and then fired (cured) using radiant heat sources. This is a slow process with limited throughput and involves considerable energy consumption. In another approach, described in U.S. Pat. No.5,720,859 (Raychem), a combination of a metal source and a source of reducing carbon are coated onto a varistor body. The coated varistor is then irradiated with microwave radiation to effect carbothermic reduction of the metal source to metal.
The present invention is, directed towards providing an improved production process which involves less energy consumption, and a shorter time.
According to the invention, there is provided a method of producing a passive device comprising the steps of pressing a ceramic powder to provide device bodies, sintering the device bodies, depositing outer electrodes onto the device bodies, and firing the outer electrodes, characterised in that,
the device bodies are sintered by irradiation with microwave radiation in which:
the temperature of the device bodies is increased in a ramping stage to a maximum temperature in the range of 1000xc2x0 C. to 1300xc2x0 C. and is then maintained at substantially the maximum temperature; and
the time duration for both the ramping stage and maintenance of the temperature at substantially the maximum temperature is in the range of 120 minutes to 180 minutes.
In one embodiment, at least one thousand device bodies are mounted within a container comprising insulating outer walls and internal heat distribution rods, and the container is irradiated with said microwave radiation.
In one embodiment, the outer walls are of a fibrous ceramic material.
In another embodiment, the heat distribution rods are of silicon carbide material.
In one embodiment, the heat distribution rods are of molysilicide material.
In one embodiment, the temperature ramping gradient is in the range of 10xc2x0 C. to 20xc2x0 C. per minute.
In one embodiment, the gradient is in the range of 15xc2x0 C. to 16xc2x0 C. per minute.
In one embodiment, the device body comprises a multi-layer structure of ceramic layers separated by inner electrodes.
In a further embodiment, the inner electrodes are of platinum material.
In one embodiment, the inner electrodes are applied by wet stacking.
In one embodiment, the outer electrodes are fired by irradiation with microwave radiation.
In one embodiment, the radiation is applied to heat the outer electrodes to a temperature in the range of 550xc2x0 C. to 820xc2x0 C. in a time for temperature ramping and maintenance of 30 minutes to 45 minutes.
In one embodiment, the ramping gradient is in the range of 20xc2x0 C. to 25xc2x0 C. per minute.
In one embodiment, the outer electrode material is silver-based.
According to another aspect, the invention provides a method for applying an outer electrode to the body of a passive device, the process comprising the steps of:
applying electrode material to the body as a film; and
firing the electrode material by subjecting it to microwave radiation.
In one embodiment, the firing temperature is within the range of 550xc2x0 C. to 820xc2x0 C. and the firing time period is, 10 to 20 minutes.
In one embodiment, the firing time is approximately 15 minutes.
In one embodiment, the total time duration for temperature ramping and maintenance at the firing temperature is in the range of 30 to 45 minutes.
In one embodiment, the outer electrode material is silver-based.